Gamma-ray bursts are short and intense flashes of energetic radiation emitted from outer space. These bursts are the brightest explosions in the universe. The Vela satellites—defense satellites that monitor human-induced nuclear explosions in space—accidentally discovered the bursts in the late 1960s since the atmosphere blocks the gamma-rays.
From the time the bursts were discovered, they have been the center of attention with a number of dedicated satellites launched to investigate their origin. In the late 1990s, it was found that long bursts (that last more than a few seconds) are caused by the death and disintegration of enormous stars. By contrast, in the first decade of the 21st century, it was realized that shorter bursts (that last less than a few seconds) are caused by the merger of neutron stars.
This last discovery was spectacularly confirmed before two years with simultaneous detection of gravitational waves by the gravitational wave detectors Virgo and LIGO, as well as a short burst by two satellites—ESA’s INTEGRAL and NASA’s Fermi.
Several other mysteries related to these bursts that remained, specifically, perplexing was how the high energy radiation is generated. In January 2019, a gamma-ray detector onboard NASA’s Neil Gehrls Swift satellite detected GRB190114C, a bright burst that occurred 4.5 billion years ago in a distant galaxy.
After a trigger from Swift—The MAGIC telescope—a Cherenkov detector located at the Roque de los Muchachos Observatory in La Palma, Spain swerved toward the location of the burst and observed photons with extremely high energy (of the order of several TeV) radiating from it.
The ultra-high energy TeV photons that were detected nearly 50 seconds following the prompt emission, in what is called the “afterglow” phase, were at least 10 times more energetic compared to the highest energy photons observed earlier from any burst.
So far, only preliminary data related to the MAGIC observations have been posted. Yet, Prof. Evgeny Derishev from the Institute for Applied Physics in Nizhny Novgorod and Prof. Tsvi Piran from the Hebrew University of Jerusalem integrated the data with observations of lower energy (X-ray) photons performed by the Neil Gehrels Swift and have demonstrated that they unravel the details of the mechanism behind the emission.
In a paper published in the Astrophysical Journal Letters on August 1st, 2019, the researchers demonstrate that the origin of the observed radiation must be a jet moving at 0.9999 of the speed of light toward the Earth. The high-energy radiation detected by MAGIC was emitted by electrons accelerated to TeV energies inside the jet.
It is possible to identify the emission process, which is the so-called “inverse Compton mechanism” where ultra-high energy electrons bump into low-energy photons and boost their energy. Strikingly, the same relativistic electrons also produced the low-energy “seed” photons through synchrotron radiation.
MAGIC has found theRossetastone of gamma-ray bursts. This unique detection enables us for the first time to discriminate between different emission models and discover what are the exact conditions in the explosion. We can also understand now why such radiation wasn’t observed in the past.
Tsvi Piran, Professor, Hebrew University of Jerusalem
When compared to MAGIC, future Cherenkov telescopes—for example, the envisioned Cherenkov Telescope Array, which is a multinational project that is being built now—will be even more sensitive. The current observation indicates that many such events will be observed in the future and will continue to offer insights into the cosmic mystery.